apple photos app messed up dates

There is no command to get all selected photo the same date.

The command "Image > Adjust date and time" is meant to be used to correct a wrong clock setting in the camera. It will shift the date and time of all selected photos by the same offset, but not set them to the same date and time.

When I am setting the dates and times of a bunch of items to the same date, I am using an Apple Script, that will copy the date and time of the first selected photo to all other selected photos and add an offset of one second between successive photos, so they can be sorted by the same and keep the same sort order.

See: Script: Batch Change the Date and Time to a Fixed Date

Or, if you want to copy the title and location as well: Script: Lift and Stamp all Metadata from one Photo to All Other Selected Photos

Where are you seeing these dates? Is this all images or a subset? Is it when you import new images?

Not enough info for an answer.